Integrated Fungicide Programs for Early Season Apple Disease Control

Early season season disease control on apple, which is defined in this article as the period from 1/2″ green to second cover, consists of simultaneous management of apple scab, powdery mildew, and cedar apple rust. Of these three diseases, the primary target is typically apple scab. During this period, the initial scab inoculum in the form of ascospores infects both leaves and young fruit. 

Four new fungicides (chemistry) became available this year for use in apple orchards in New Jersey:  Fontelis (SDHI), Luna Sensation (SDHI+QoI), Luna Tranquility (SDHI+AP), and Merivon (SDHI+QoI). These materials join another fairly recent addition, Inspire Super (DMI+AP), and a somewhat older but related fungicide, Pristine (SDHI+QoI).

The question is ” How do we integrate these new fungicides into our current programs?” and perhaps more importantly, “How can we use them to improve fungicide resistance managment in our orchards?”. Yes, program efficacy across the three diseases is still important, but resistance management is paramount to sustainability. On a variety of crops, plant pathogens have increasingly become resistant to not just one, but two chemistries. Perhaps, using chemistries in succession over the years, rather than together in the same program, allows pathogens to accumulate resistance genes, especially since in many cases the resistance does not reduce overall fitness.

Foundation programs. Development of integrated fungicide programs requires two steps. The first step begins with creation of “Foundation Programs”. These programs consist of a protectant fungicide (Prot) tank mixed with an at-risk systemic fungicide of a single chemistry. This should be a familar tactic.The protectant fungicides are one the EBDCs (mancozeb or Polyram) or captan. If using an EBDC, it is added at half-rate to allow use of the extended schedule through second cover. Given current available fungicides for apple, the following table shows four possible foundation programs:

In these programs, the DMI fungicide can be Indar, Procure, Rally, Vintage, or Topguard (group 3); the AP fungicide can be Vangard or Scala (group 9); the QoI can be Sovran or Flint (group 11); and the SDHI is Fontelis (group 7). You might recognize some of these possible programs, such as Rally + Captan, as THE program that was used for early season disease control. However, in this case, these foundation programs are just that – foundations – upon which to build an integrated program. They are not meant to be applied alone.

Integrated programs. The second step in creating integrated programs is to substitute one of the newer fungicides that contain two active ingredients of different chemistry into the foundation programs at the tight cluster (TC), bloom (B), and first cover (1C) timings.  The rule to follow is that each of the two chemistries in the new fungicide must be different from the chemistry of the at-risk foundation fungicide. When performing this operation, a “family” of six integrated programs is created (foundation spray mixtures in green; newer two active ingredient fungicides in yellow):

The guiding principle or resistance managment strategy behind these programs is that two different chemistries are always being applied. In the case of the newer two-active ingredient fungicides, the pathogen is being forced to undergo two simultaneous mutations to each active compound, an event of much lower probability.The alternation of the chemistries and use of a protectant adds to the overall stength of the programs.

Program selection. All six integrated programs are not equal in their overall efficacy or ability to control all three diseases, nor is this necessary. Determining which program to apply is dependent on the needs of the orchard block and the cost of the program.

By examining fungicide efficacy tables, preferably from at least two sources (easy to do online), each program can be given a rating for control of each of the three diseases. The overall cost of each program can also be determined from current prices. One then selects the least expensive program that provides the control spectrum needed for the block. For example, if a cultivar is not susceptible to powdery mildew, a “fair” powdery mildew rating would be acceptable.

Program modifications. The integrated programs could be altered in two ways. First, a protectant could be mixed with the newer fungicides containing the two active ingredients. This approach is probably not necessary, but could be considered.

Second, the newer two active-ingredient fungicides could be applied four times, at 1/2″G, P, PF, and 2C, with the foundation mixture alternating in-between. Depending on the individual fungicide efficacy ratings, this change may or may not improve overall program efficacy. This modifcation would certainly be more expensive, but allowable. All of the newer fungicides can be applied up to four applications per season.

Rates and coverage. If the label for an at-risk fungicide (single or two active ingredients) lists a rate range, then application of a middle-rate is suggested as the starting point. This rate can be adusted accordingly as one gains experience with the product and program, or to compensate for favorable conditions, inoculum levels, etc… Unless results of field trials indicate otherwise, use the low end of the rate range with caution.

When spraying alternate middles, both middles should be sprayed with the same mixture before alternating to the next mixture in the program. Also, remember that alternate middle sprays should be applied twice as often. A 7-day program is applied to half the orchard every 3.5 days, a 10-day program every 5 days, etc…

Inoculum levels. Pre-season management of scab inoculum levels should always be considered an integral part of the disease control program. Fall or early spring (late dormant) application of urea should be applied to the orchard floor to reduce overwintering ascosporic inoculum. Or the leaves [from last year] could be mechanically shredded. Also, copper applications should be applied at bud-break.

Prior resistance. Ideally, the integrated programs are best deployed when resistance does not already exist in an orchard. Under these circumstances, the three different chemistries and protectant all work in synchrony ro reduce the likelihood of selection for resistant strains.

If resistance to one of the chemistries is known or suspected, then one could theoretically employ those programs excluding that chemistry. For example, if DMI-resistant scab strains exist in an orchard, then one of the two integrated programs that lack a DMI fungicide could be utilized (programs 2 or 6).  If resistance exists to more than one chemistry, then a protectant only program may be the only choice.

Caveats. Cost of the newer fungicides will most likely be the biggest deterrent to implementing these integrated programs. Furthermore, having to purchase and manipulate multiple products is an inconvenience and adds to the up-front costs.

Regardless of these caveats, sole application of at-risk single-active fungicides with protectants does not appear to be a strategy that is sustainable. Given our understanding of the evolution of fungicide resistance, employing multiple chemistries in the same program, both as mixtures and in alternation, appears to make the most sense.